2008 Chrysler 300 not running right

My son has a 2008 300 with a 3.5l engine with over a 150K on it. He says it's not running right and in limp mode. We stuck my scan tool on it and do not see any codes. He said it could be one of three things: the switch or sensor on the accelerator pedal, the throttle body sensor, or what he is calling a Flashmaster engine computer located under the hood near the passenger side hood hinge (mounted to a cross member holding the dust shield for wipers). Can anyone give me direction? Thanks.

. . . .My son has a 2008 300 with a 3.5l engine with over a 150K on it. He says it's not running right and in limp mode. . . .

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If you could further define "limp mode" it would help greatly. Is the transmission only in 2nd gear and is not shifting? That would be transmission limp mode.

If you press the accelerator pedal and engine rpm increases to a low limit like 1000 rpm then that is throttle control limp mode. If that is the issue then you would have a warning indicator as follows on the instrument panel.

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A transmission (TCM) 2nd gear limp-in should set a fault code. Since TCM faults aren't necessarily 'OBDII' (emissions), some engine code readers won't read them. Some can only read PCM codes.
I wouldn't replace anything until more diagnosis can be done. Try another scan tool? Is the 'ck eng' or ETC (throttle) light on?
By 'not running right', do you mean that there is a misfire or hesitation or it stays in one gear? Need more info.